1. Field of the Invention
The present invention relates to restoration of load transfer capacity in horizontal concrete slabs, particularly restoration of load transfer capacity in horizontal concrete slabs by retrofit mechanical reinforcement of control and other joints.
2. Description of the Prior Art
The pouring of large horizontal concrete slabs on top of “grade” or graded earth or gravel, without any reinforcement steel bar (or “rebar”), is a fairly common practice. This practice is used both for outside pavement and indoor flooring on which wheeled traffic is expected. The load imparted by wheeled traffic to “on-grade” concrete slabs is predominantly compressive. Since concrete has excellent compressive strength, the use of reinforcement steel bar in many on-grade slab applications has often been considered unnecessary. Roads, airport pavements and warehouse flooring are a few common examples of areas in which large horizontal concrete slabs without steel reinforcement have been poured.
Rather than reinforce large horizontal concrete slabs poured on top of grade, “control joints” are commonly used. Control joints are straight grooves made on concrete slabs in order to “control” where the concrete should crack (due, for example, to thermal expansion and contraction, or due to uneven curing rate of the poured concrete). Control joints are commonly effected by superficial divisions or scoring of the concrete, typically created either by molds during the pour (in which case the molds are removed before the concrete fully sets), or (more typically in larger concrete slabs) by cuts with radial saws after the concrete sets up. Control joints ensure that fractures, caused by contraction during curing of large horizontal concrete slabs, occur only in regular, predictable patterns and locations.
Other common construction joints (for example joints at the interface of adjacent concrete slabs, or between a slab and a wall) typically extend the “full depth” of the concrete slab. Such full depth joints extend from the top of the concrete slab to the bottom of the concrete slab, corresponding to the depth of the concrete pour and the form in which the concrete was poured.
Control joints (after fracturing) and full depth joints are both referred to herein as “loose joints”. As used herein, the term “loose joint” refers to any substantially vertical break in a concrete slab, wherein the vertical break extends from the bottom of the slab to the top of the slab, producing two adjacent slab sections that can, under certain circumstances (including, for example, the application of sufficient force), move substantially independently of each other. Loose joints are often found in grid patterns, or, in the case of roads or lanes of a highway, spaced apart at regular intervals, parallel to each other and perpendicular to the direction of traffic thereupon.
Concrete pavement, including roads, highways, alleys, parking lots, loading docks, airport runways, taxiways, and aprons, is also subject to damage from the elements. Damage caused by freezing rain to concrete slabs that are exposed to the weather is well known, as is damage to concrete slabs caused by thermal expansion and contraction of the concrete slab sections themselves. Joint filler is often used to seal cracks that are caused by these conditions. However, application of joint filler, alone, is of virtually no use in preventing further damage to the slab in the vicinity of (sealed) cracks that may be caused by heavy loads imparted by vehicular traffic to the slab in the vicinity of the (sealed) cracks.
The loads that are imparted to concrete slabs by vehicular traffic may be substantially unidirectional (such as in the case of separately poured lanes of a road or highway), substantially bi-directional (such as in the case of two-way avenues, including airport runways and taxiways, alleys, one lane roads, and the flooring in the aisles of distribution warehouses), or random (such as in the case of airport aprons and in areas in front of or between loading docks). Loose joints are most commonly aligned in concrete slabs perpendicular to the direction of vehicular traffic flow over the slab.
It will be appreciated that, in addition to being sheltered and, therefore, protected from damage caused by freezing water, (indoor) flooring typically is not exposed to as wide a range of ambient temperatures as outdoor pavement, and, accordingly, is not subjected to as much thermal expansion- and contraction-induced stress as outdoor pavement. Nonetheless, indoor concrete flooring slabs are vulnerable to thermal expansion- and contraction-induced cracking.
Poured concrete contracts as it cures. Control joints in indoor concrete floors that are produced by cracking of the concrete slab during the concrete's curing are generally sufficient to accommodate anticipated (horizontal) movement of the slab sections resulting from thermal expansion.
However, as on-grade concrete slabs continue to cure, the edges (particularly the opposed edges of slab sections at loose joints) curl up. Concrete continues to cure over time, at an ever decreasing rate, with a full cure typically considered to require two years. Even when only subjected to substantially vertical compressive loads (for example, due to heavy vehicular traffic over the slab), this results in damage to the slab section edges, and often causes progressively worse fracturing of these areas.
In addition, curling (of the slab section edges) by itself can present a problem for certain types of vehicles that may travel over the slab. For example, the flooring in very narrow aisles between tall shelves in large distribution warehouses are often used by tall fork lifts, which travel over the flooring and require a very flat surface for proper operation. As a forklift nears the end of one (concrete) flooring section, its weight forces the edge of that section of flooring down, exposing a vertical edge of the adjacent section of (concrete) flooring. This phenomenon causes a temporary bump in the flooring, which can result in damage to the forklift, mishandling of the forklift and/or accelerated deterioration of the flooring. The same phenomenon is observed in pavement with other types of wheeled vehicles. Use of large pneumatic tires on the wheels of most vehicles such as trucks, cars, and airplanes can, in some instances, mitigate the effects of this phenomenon, but it does not eliminate the effects, particularly if the floor or pavement is subjected to heavy traffic.
To address some of the above-described problems, various approaches have been proposed and used in the prior art.
Joint filling has been mentioned above as one proposed prior remedy. Typically a semi-rigid epoxy or polyurea compound is utilized. This prior method is relatively easy, and is somewhat effective in preventing damage caused by freezing water. However, this prior method is quite ineffective in preventing deterioration of section edges of on-grade concrete slabs that is caused by loading (for example, by vehicular traffic) of the slab.
A second prior approach attempts to provide vertical support to the concrete section edges by filling the gaps (between the bottom of the slab and the ground) that often develop under the edges of the concrete slab sections. In this prior method, the gaps underneath the edges of the concrete slab sections are (attempted to be) filled with grout. The prior practice of sub-slab grout injection is often known colloquially as “mud-jacking”. A grout consisting of water diluted Portland cement is pumped into (vertical) holes that are first drilled all the way though adjacent concrete slab sections along both sides of the crack or joint. This, in theory, and in combination with grinding of the tops of the curled edges, repairs the damage and purports to prevent further deterioration of the loose joints that are so repaired. In practice, however, it is extremely difficult to fully fill the voids beneath the edges of the concrete slab sections, and it is virtually impossible to prevent the formation of additional sub-slab voids, which may be caused, for example, by ground settlement, water seeping between the concrete slab and grade, and continued (upward) curling of the concrete slab edge.
In a third prior approach, a strip of the concrete slab is cut out, removed, and replaced from both sides of the loose joint. Holes are also typically drilled horizontally into the new exposed edges of the cavity, and steel dowels are inserted and cemented in place prior to filling the cavity created with new concrete.
A fourth prior approach also uses steel dowels that are cemented into substantially horizontal slots cut into two adjacent slab sections, normal to the loose joint and parallel to each other. The slots are then filled with a strong grout.
Lastly, as an alternative to the four above-mentioned prior remedies, there is slab replacement. In this prior method, substantial portions, if not the entirety, of a concrete slab floor or pavement is removed and replaced with a new concrete slab.
The fact that slab replacement is performed is indicative of the limited effectiveness of the known alternatives in the prior art for preventing and repairing deterioration of loose joints in large horizontal concrete slabs. Replacement of the entire slab of concrete obviously causes great disruption of traffic and other use of the affected floor or pavement. All of the aforementioned prior methods involve a considerable amount of work and disruption of use of the affected pavement/floor. In each of the prior methods that involve the removal and subsequent replacement of a strip of concrete, time for setting and hardening of the new concrete adds to the time before which the floor or pavement can be reused.
A critical problem with most prior methods is the inability to transfer loads across loose joints of adjacent sections of concrete pavement or flooring. This is why prior methods of sealing joints or cracks, while somewhat effective against damage from freezing water, is wholly ineffective against deterioration caused by traffic upon the surface across loose joints. This is also why prior full-depth partial slab replacement methods (without steel dowels) result in exacerbating the problem, by inherently doubling the number of joints that exist. Prior methods that involve retrofitting of steel dowels result in constructions that tend to resist relative (vertical) displacement of adjacent edges of concrete sections. However, vertical loads (for example, as applied by heavy vehicular traffic) that are transferred from one slab section to the next are concentrated upon the dowel-concrete interfaces (or dowel-cement interfaces) directly above the dowels on one side of the loose joint, and directly underneath the dowels on the other side of the loose joint. This results in highly concentrated shear forces in the concrete in the vicinity of the dowels, which often leads to fractures in the concrete and/or cement.